Three thin, concentric, conducting spherical shells have radii \(R\), \(2R\), and \(3R\), and carry net charges \(+q\), \(+2q\), and \(-3q\), respectively, as shown. Assuming the electric potential is defined to be zero at infinity (\(V(\infty) = 0\)), what is the electric potential \(V\) at the surface of the middle shell at a distance \(r = 2R\) from the center?